Preclinical studies aimed at improving and optimizing various physical and chemical aspects of boron neutron capture therapy are being conducted. The Medical Therapy Facility of the MIT research reactor (MITR-II) has been refurbished for a new clinical trial. This includes an optimum thickness of bismuth shielding in the beam, a new patient collimator and collateral shielding the produce a well thermalized neutron beam with a reduced gamma ray and negligible fast neutron contaminations. A rapid boron assay technique to measure small concentrations of 10B in blood (0-50 micrograms 10B per gm) is also being perfected. Patient studies are being conducted at the Massachusetts General Hospital to measure the relative uptake of Na2B12H11SH in tumor and blood in patients with high grade astrocytomas. Animal irradiation studies aimed at measuring the RBE of boron neutron capture therapy are being carried out.